Invited speaker: Tommaso RoscildeAffiliation: ENS LyonTitle: Quantum correlations: Where are they? How do they build up? How to measure them?

Time and room: 17:15 h, lecture hall IAP

When a system is composed of many parts, "more is different" (as P. W. Anderson famously wrote) if and only if there are correlations among the various parts. If the constituents are quantum mechanical atoms, spins, oscillators, etc., "more" can be even "more different", as correlations can take forms which are impossible in classical mechanics. The most famous, yet elusive form of quantum correlation is represented by entanglement, a property well defined and investigated for pure states, and envisioned as a resource for nearly all technological tasks harnessing quantum many-body systems. In the real life of mixed states incoherent fluctuations appear in the game, making the distinction of quantum vs. classical correlations less sharp. At the same time, the exquisite level of control achieved by experiments in atomic, molecular and optical (AMO) physics enables nowadays to engineer correlated phases of quantum many-body systems, so that the ability to characterize and control quantum correlations becomes a fundamental question, as well as (possibly) a technological one.
In this colloquium I will try to offer a broad overview of the theoretical importance of quantum correlations, starting from their very definition - to which we contributed recently with a statistical physics approach which allows to calculate them in generic systems, and potentially to measure them for a large class of quantum many-body systems relevant to experiments in AMO physics and beyond. I will moreover discuss the centrality of quantum correlations in the dynamics that leads a closed quantum system to relax to an equilibrium state, contrasting the case of short-range interactions with that of long-range ones: this contrast allows to enlighten the role of elementary excitations (and in particular of their dispersion relation) as the "carriers" of quantum correlations.

Spatio-temporal confinement of light can dramatically enhance light-matter interactions. To achieve this capability on an accessible platform, we have developed microscopic Fabry-Perot cavities based on laser-machined optical fibers [1].
We employ such cavities to realize efficient and narrow-band single-photon sources by means of Purcell enhancement of fluorescence emission. We study color centers in diamond such as the Nitrogen-Vacancy center [2] and explore different regimes of the cavity enhancement, aiming at applications in quantum cryptography, all-optical quantum computation, and efficient spin-state readout.
In the context of sensitive microscopy, we use microcavities for imaging and spectroscopy applications. We have developed scanning cavity microscopy as a versatile method for spatially and spectrally resolved maps of various optical properties of a sample with ultra-high sensitivity. We demonstrate the technique by quantitative imaging of the extinction cross-section of gold nanoparticles and measurements of the birefringence and extinction contrast of gold nanorods [3]. Finally, we show that the Purcell effect can be used for cavity-enhanced Raman spectroscopy and hyperspectral imaging [4]. Simultaneous enhancement of absorptive, dispersive, and scattering signals promises intriguing potential for optical studies of nanomaterials, molecules and biological nanosystems.

Qubits based on electron spins trapped in fabricated semiconductor nanostructures such as gate controlled quantum dots or individual donors are considered promising candidates for scalable solid state quantum computing, which promises an exponential speedup for certain computational tasks.
Starting from an outline of the development of key concepts, I will give a (selective) overview of the current state of the art of GaAs and Si based spin qubit devices. Key results include single shot readout, high fidelity single-qubit manipulation, and first demonstrations of two-qubit gates. One particular focus of my talk will be effects from the hyperfine interaction of nuclear spins with the electron spin qubit, which is a source of strong decoherence when present and unavoidable in GaAs devices. The intricate physics emerging from hyperfine coupling is now rather well understood and we have developed effective methods to reduce the associated dephasing and achieve high fidelity qubit control.

Topological insulators and superconductors are new quantum states of matter that are characterized by nontrivial topological structures of the Hilbert space [1]. Recently, they attract a lot of attention because of the appearance of exotic quasiparticles such as spin-momentum-locked Dirac fermions or Majorana fermions on their surfaces, which hold promise for various novel applications [2]. In this talk, I will introduce the basics of those materials and present some of the key contributions we have made in this new frontier.
[1] Y. Ando, Topological Insulator Materials, J. Phys. Soc. Jpn. 81, 102001 (2013).
[2] Y. Ando and L. Fu, Topological Crystalline Insulators and Topological Superconductors: From Concepts to Materials, Annu. Rev. Condens. Mater Phys. 6, 361 (2015).

In this talk, I will first give a brief introduction to the field of cavity optomechanics, where one couples radiation fields
to the motion of mechanical resonators. I will then explain how optomechanical interactions can be exploited to modify the transport
of phonons and photons in two-dimensional arrays of coupled optical and vibrational modes. These can e.g. be implemented in photonic crystal slabs.
Engineering the light field wave front, it is possible to generate a topologically nontrivial bandstructure via the optomechanical interaction.
This gives rise to transport of sound waves along chiral edge channels that are robust against disorder. In the last part of the talk, I will
indicate how one can even use a purely geometrical nanoscale design for chiral sound wave transport in a pseudo-magnetic field.